In recent years, reduction of amounts of treated wastewater is earnestly demanded in the field of medical services from the standpoint of environment protection and space saving. According thereto, techniques relating to such photosensitive thermal developing photographic materials for medical diagnosis and photographic techniques are required that can be efficiently exposed in a laser image setter or a laser imager to form a sharp black image having high resolution and sharpness. In these photosensitive thermal developing photographic materials, the use of a solvent series treating chemical reagent can be eliminated to provide customers with such a simple thermal developing system that does not impair environments.
While there are the similar demands in the ordinary image forming materials, a medical image has such characteristics that high image quality with excellent sharpness and graininess is demanded owing to minute depiction, and further, a cold tone image is preferred owing to easiness in diagnosis. Under the current circumstances, various kinds of general hardcopy systems utilizing pigments and dyes are commercially available, such as an ink-jet printer and electrophotography, but there is no system that is sufficient as an output system for a medical image.
A thermal image formation system utilizing an organic silver salt is described, for example, in the specifications of U.S. Pat. Nos. 3,152,904 and 3,457,075, and B. Shely, “Thermally Processed Silver Systems”, Imaging Processes and Materials, Noblette 8th ed., edited by Sturge, V. Walworth and A. Shepp, p. 2 (1996).
In particular, a photothermographic material generally has a photosensitive layer containing a binder matrix having dispersed therein a photocatalyst in a catalytically active amount (such as a silver halide), a reducing agent, a reducible silver halide (such as an organic silver salt), and depending on necessity, a toning agent for controlling the tone of silver. The photothermographic material forms a black silver image through imagewise exposure and heating to a high temperature (such as 80° C. or more) to cause a redox reaction between the silver halide or the reducible silver salt (functioning as an oxidizing agent) and the reducing agent. The redox reaction is accelerated by the catalytic function of a latent image of a silver halide formed by exposure. Therefore, the black silver image is formed in the exposed region. As a medical image forming system utilizing a photothermographic material disclosed in a large number of literatures, such as U.S. Pat. No. 2,910,377 and JP-B-43-4924, Dry Imager FM-DP L has been released by Fuji Medical Systems Inc.
In the production of the thermal image forming system utilizing an organic silver salt, the photothermographic material is produced by two methods, i.e., a method of coating a coating composition containing a solution formed by dissolving a polymer of a main binder in an organic solvent, followed by drying, and a method of coating a coating composition containing polymer fine particles of a main binder as an aqueous dispersion, followed by drying. The production equipment for the later method is simple since a recovering step of a solvent is not necessary, and thus the method is advantageous for mass production.
The thermal image forming system utilizing an organic silver salt has a severe problem in storage stability of an image after developing treatment, particularly deterioration of a printout upon irradiation with light, due to the absence of a fixing step. As a method for improving the printout, a method of utilizing AgI formed by converting an organic silver salt is disclosed in U.S. Pat. No. 6,143,488 and EP 0,922,995. However, the methods of converting an organic silver salt with iodine as disclosed herein cannot provide sufficient sensitivity to fail to build a practical system.
Other examples of sensitive materials utilizing AgI are disclosed in WO97/48014, WO97/48015, U.S. Pat. No. 6,165,705, JP-A-8-297345 and Japanese Patent No. 2,785,129, but they fail to attain the sufficient sensitivity/fogging level and cannot be applied to practical use. A method for utilizing a silver halide having a large silver iodide content is being demanded.
JP-A-2000-305213 discloses an image forming process and a photosensitive material utilizing blue to ultraviolet laser light as an exposure light source, but AgI is not used therein, and the sensitivity thereof is insufficient.